Twin-wire presses for dewatering of a fiber suspension and forming of a continuous web thereof are previously known. Dewatering of the pulp is usually done from an inlet pulp concentration of from 3 to 8 percent by weight to an outlet pulp concentration of from 30 to 50 percent by weight. According to the state of the art, such twin-wire presses comprises lower rolls, an endless lower wire running in a path around the lower rolls, upper rolls, and an endless upper wire running in a path around the upper rolls. The two wires co-operate with each other along sections of said paths that run substantially in parallel with each other for dewatering of the fiber suspension between the wires during displacement thereof. An inlet box provides for supply of the fiber suspension to a wedge-shaped dewatering space between the wires. The twin-wire press further comprises two dewatering tables supporting the respective wires in said sections of the path and forming the wedge-shaped dewatering space between the wires for initially pressing and dewatering the fiber suspension, whereby a web is formed between the wires, and a roll arrangement situated after the dewatering tables in said sections of the paths, as seen in the direction of movement of the wires, for finally pressing and dewatering the web between the wires, so that the web will obtain a desired dryness. By dewatering space is meant the part of the dewatering tables where dewatering occurs.
A conventional dewatering space in a twin-wire press has a wedge-shape with a fixed shape that is not changeable when the twin-wire press is in operation. The geometry of the table and the flow of the pulp suspension creates the operating pressure difference over the wire that controls the dewatering. The wedge-shape determines the built up pressure in the twin-wire press and the dewatering procedure is to a large extent dependent on the shape of the wedge, which is difficult to change. Alteration of the wedge-shape requires new, extensive settings of the dewatering tables, exchange of side sealings to the dewatering tables, etc.
The dewatering tables in a twin-wire press is the first step in the dewatering of a fiber suspension and is adapted for relatively slow dewatering and also for preparation of the formed fiber web, formed through dewatering of the fiber suspension, for the much faster dewatering that occurs in the subsequent roll arrangement through pressing in a roll nip. Too large a loading of the dewatering tables directly results in high frictional forces and also very high energy consumption for the operation. The step between the relatively slow dewatering in the dewatering tables and the very fast dewatering in the roll arrangement is considerable and may sometimes give rise to problems in the first roll nip in the roll arrangement. In case the fiber web has far too low a pulp concentration when it leaves the dewatering tables, and is received and deformed with far too high a speed in the first roll nip in the roll arrangement, the fiber web may be destroyed.
One object of the present invention is to at least partially eliminate those drawbacks associated with the previously known state of the art of twin-wire presses that have been described above. A further object of the present invention is to achieve easier and more improved dewatering by the dewatering tables in a twin-wire press without changing the geometry of the dewatering tables. An additional object of the present invention is to overcome the considerable difference of conventional twin-wire presses between the relatively slow speed at which the fiber suspension/web is deformed and the relatively high speed the fiber web is deformed by the roll arrangement.